1. Field of the Invention
The present invention relates to a liquid crystal display device. In particular, it relates to a transmissive type liquid crystal display device provided with a back light at its rear side.
2. Discussion of Background
In the transmissive type liquid crystal display device provided with a back light at its rear side, a light shielding layer is provided at an inner side of the cell to improve visibility. FIG. 2 is a diagrammatical cross-sectional view of a typical conventional device.
In FIG. 2, a liquid crystal display device 10 comprises a pair of transparent substrates 11, 12 made glass or the like wherein a terminal portion 11a is formed on the transparent substrate 11. Transparent electrodes 111, 121 of ITO (Indium Tin Oxide) each having a predetermined display pattern are formed on the transparent substrates 11, 12 although the figure does not shown in detail. A light shielding layer 112 is provided on the transparent substrate 11 at a position excluding a portion corresponding to the display pattern. On a terminal portion 11a, there are formed a first lead-out electrode group 131 drawn directly from the transparent electrode 111 of the transparent substrate 11 and a second lead-out electrode group 132 connected to the transparent electrode 121 from the other transparent substrate 12.
The transparent substrates 11, 12 are bonded by interposing a periphery sealing member 14 made of an epoxy resin or the like so that the transparent electrodes 111, 121 oppose to each other. Liquid crystal 15 is sealingly put in a cell gap thus produced. The periphery sealing member 14 includes a transfer material of, for instance, conductive beads, and the transparent electrode 121 on the other transparent substrate 12 is connected electrically to the second lead-out electrode group 132 on the terminal portion 11a by means of the transfer material.
Polarizers 161, 162 are disposed at outer surface sides of the transparent substrates 11, 12. In this liquid crystal display device 10, the transparent substrate 11 having the light shielding layer 112 is determined as a rear side, and a back light 17 is provided in rear of the transparent substrate 11. Accordingly, the transparent substrate 12 is to be a top surface side.
In forming the above-mentioned light shielding layer, a printing method was mainly employed to the conventional device. However, in order to obtain a sufficient shielding property of 2 or more in terms of an optical density (OD value) by the printing method, the film thickness of the light shielding layer reached 3xe2x80x944 xcexcm. Accordingly, there was such drawback that smoothness in the top surface was poor and fine processing could not be performed due to a restriction in accuracy of printing. Accordingly, in an attempt of forming the cell gap to be small, it was only possible to reduce the dimension of the gap to about 6 xcexcm at most due to the restriction on the film thickness by the printing. Further, even in this case, a short-circuit between the opposing transparent substrates and unevenness of the gap often occurred due to protections formed in the light shielding layer by which there were problems of bad yield or a poor quality of products.
Further, as methods for realizing a partial color display by using the cell provided with the light shielding layer at its rear side, there were a method for conducting color-printing to an outer portion of the cell by screen printing, a method for partially coloring by disposing a color filter in an outside portion of the cell and a method for printing a color layer on an inner surface of the cell by screen printing or off-set printing. However, there were problems as follows.
In the method for printing or attaching a color filter to an outer surface of the cell, it was impossible to avoid color shifting due to parallax, and the application of different colors in a fine pattern was difficult, whereby it was only possible to form a rough pattern having the intervals of line of about 1 mm or more. Further, in the method for disposing a color filter in the cell by printing, the printing should be conducted on the ITO; transparent electrode, and accordingly, a voltage was applied to the liquid crystal layer through the color filter, which deteriorated the threshold property. Further, there was a method for changing colors to be displayed by forming sections having different colors in a color polarizer. However, this method was too expensive to use.
On the other hand, there has been well known a technique to prepare a color filter with use of a black resist and a color resin, which has become practical for a full dot display such as a large sized display using STN (Super Twisted Nematic), TFT (Thin Film Transistor) or the like.
The light shielding layer used for the color filter for TFTs does not require electrical insulation properties, and it is rather preferred to have a certain degree of conductive properties. It is because when an ITO film is formed on the color filter, the conductive properties of the light shielding layer compensate the ITO film having insufficient conductive properties and exhibit excellent operating characteristics even though the ITO film has a relatively high resistance value.
On the other hand, in a case of using a color filter for STN in which an ITO film having a stripe-like patterning on the color filter, it is not preferable to have conductive properties from the viewpoints of assuring electrical insulation properties between lines and reduction in the electrostatic capacity. However, the smoothness of the top surface is an important factor for the color filter used for STN, and therefore, it is necessary to form a insulating smoothing layer of resin on the color filter.
Since the resin having a insulating smoothing layer has electrical insulation properties, use of the light shielding layer having low electrical insulation properties on the color filter does not create a big problem. Namely, although a certain material having high insulation properties and high light absorbance was known, persons have not found any usage to the material having such excellent characteristics.
Further, the bonding strength between the transparent substrate and the light shielding layer formed by printing was insufficient, and it was impossible to print the periphery sealing member on the light shielding layer. Accordingly, when a display portion of a liquid crystal panel mounted on an automobile is observed from an oblique direction and if the angle of visibility is too deep to be out of the light shielding layer, the light of the back light may leak.
In order to prevent such disadvantage, a black ink was printed on a region of a top surface of the cell including a portion corresponding to the periphery sealing member into a flame like shape so as to prevent the leakage of light. However, this technique inevitably increased the number of processes. Besides the abovementioned problem, there was proposed a color display system to display different colors by driving the cell at a low duty ratio. However, this technique had the problem as follows. Namely, in using the method for printing the color filter or bonding the color filter on an outer portion of the cell, it is necessary to broaden the distance between patterned portions in order to prevent colors from mixing due to parallax, and accordingly, it is impossible to provide precisely divided colors.
On the other hand, in the method for printing the color filter in an inner surface of the cell, the color purity was poor although there was no problem in precision, and reliability was low. Even in the method for printing the color filter in an inner surface of the cell, it was necessary for the light shielding layer formed by printing to have a film thickness of 2-4 xcexcm to obtain a sufficient absorbance, and therefore, the production of an uneven film thickness was unavoidable. Accordingly, an unevenness of cell gap was resulted whereby there was a scattering of characteristics on final products. Further, there was a limit on precision in using the printing method; it was very difficult to design a pattern as fine as 200 xcexcm or lower when different color should be provided in each side, and there was also a restriction in design. Further, when a pre-tilt angle is 2xc2x0 or less in a twisted nematic liquid crystal, a domain is produced due to the unevenness of the inner surface of the cell. Accordingly, a reduction of the pre-tilt angle for the purpose of improving the characteristics could not be realized. Further, since the smoothness of the top surface of the conventional cell was poor, the STN system or a ferrodielectric or antiferrodielectric liquid crystal system could not be employed although the driving with a duty of about 1/30 was in fact desired.
Further, when a thermoset resin was used for the periphery sealing member, a central portion of the cell was apt to be in a swelling state after the curing of the periphery sealing member due to a difference of the thermal expansion coefficient between the sealing member and the glass substrate. Even though liquid crystal sealingly injected into the cell, the swelling state of a central portion remains as it is, whereby there may be differences in the characteristics and a background color between the central portion and peripheral portions.
In order to prevent such disadvantage concerning, in particular, a large-sized panel or a STN system wherein the unevenness of the cell gap affecting adversely the optical characteristics, it was necessary to conduct pressure-sealing operation. Namely, liquid crystal was injected into the cell. The liquid crystal layer was in a pressure reducing state and was sealed under the condition that the gap in the central portion and the peripheral portion of the cell was made uniform.
Further, in a liquid crystal display device using a smectic liquid crystal such as a ferrodielectric liquid crystal or an antiferrodielectric liquid crystal, the liquid crystal may not be used again due to a stress after the sealing, an impact or vibrations because the liquid crystal does not have a self-recovering property on orientation. Accordingly, when the orientation was once disturbed, it was necessary to conduct a heating treatment depending on a degree of disturbance to thereby obtain a desired orientation.
In order to solve the above-mentioned problems, there is a method for providing a resin layer having heat softening properties around spacers and fuse-bonding upper and lower substrates at the time of curing the periphery sealing member. This method, however, had such problems that it was necessary to increase an amount of the spacers and an amount of the heat-softening resin to obtain a sufficient fixing effect, which might cause leakage of light due to the scattering and gathering of the spacers themselves and a deterioration of appearance. Further, the components of the heat-softening resin might resolve to the liquid crystal to thereby reduce the reliability.
It is an object of the present invention to provide a liquid crystal display device with a light shielding layer which provides an excellent quality of display and an excellent appearance of the product.
In accordance with the present invention, there is provided in a transmissive type liquid crystal display device comprising a liquid crystal layer, a pair of transparent substrates opposed to each other and bonded to a periphery sealing member so as to interpose the liquid crystal layer, a polarizer placed at an outer surface of each of the transparent substrates, an illumination means provided in rear of a polarizer at a back surface side with respect to a display surface side, a light shielding layer provided in an inner surface of one of the transparent substrates at positions corresponding to a non-display region and a non-display portion excluding a portion corresponding to a display pattern in a display region, and a transparent electrode formed on the display region, wherein a voltage sufficient to activate the liquid crystal layer is applied to a desired portion of the transparent electrode on the display pattern, said liquid crystal display device being characterized in that: the non-display portion in the display region has a plurality of sections of not less than 1 mmxc3x971 mm; the light shielding layer is made of a,photosensitive light shielding resin in which patterning is conducted, which has the electrical insulation property of not less than 1012 xcexa9/xe2x96xa1 (surface resistance) in terms of insulation resistance and an optical density (OD value) of not less than 2.0 per a film thickness of 1 xcexcm.
According to the present invention, the transparent electrode can be formed directly on the light shielding layer without interposing an insulating smoothing layer.
As a characteristic feature according to the present invention, the light shielding layer in the display region has a plurality of sections of not less than 1 mmxc3x971 mm and a support column made of the same material as the peripheral sealing member is provided on at least a part of the light shielding layer.
As a typical example of a display pattern wherein a non-display portion in the display region has a plurality of sections of not less than 1 mmxc3x971 mm, there is a segment type pattern such as, for example, a pattern of seven segments numeric display. However, there is another example of pattern in which a dot pattern is mixed therewith, for example, a section of 5xc3x977 dots is repeatedly provided with certain spaces.
As a characteristic feature of the present invention, a light transmitting film is made of photosensitive resin in which patterning is conducted, which light transmitting film permitting a predetermined color of light to pass therethrough is formed to cover a portion corresponding to the display pattern, and the light transmitting film is made of a resinous material having an insulation resistance of not less than 1012 xcexa9/xe2x96xa1. According to the present invention, the transparent electrode can be formed directly on the light shielding layer and light transmitting layer without interposing a insulating smoothing layer.
In accordance with an aspect of the present invention, the liquid crystal layer is a nematic liquid crystal layer having a twist angle of about 90xc2x0 between the transparent substrates; said pair of polarizers are so arranged that their polarizing axes are in parallel, and the pre-tilt angle formed between the liquid crystal layer and the transparent substrates is 1.5xc2x0 or less.
In the above-mentioned aspects of the present invention, the liquid crystal layer is a nematic liquid crystal layer, and a duty ratio for driving the nematic liquid crystal layer is 1/1-1/33.
In accordance with an aspect of the present invention, the liquid crystal layer is a nematic liquid crystal layer having a twist angle of 90xc2x0 between the transparent substrates; said pair of polarizers are arranged so that their polarizing axes intersect at a right angle, and the product xcex94nd of the refractive index anisotropy xcex94n of the nematic liquid crystal and the distance d between the transparent conductive films which form the display pattern is in a range of 4-6 xcexcm.
In accordance with an aspect of the present invention, the liquid crystal layer is a nematic liquid crystal layer having a twist angle of 70-80 degree between the transparent substrates; said pair of polarizers are arranged so that the crossing angle formed by their polarizing axes is 70-80xc2x0, and the product xcex94nd of the refractive index anisotropy xcex94n of the nematic liquid crystal and the distance between the transparent conductive films whice form the display pattern is in a range of 4-6 xcexcm.
In accordance with an aspect of the present invention, the liquid crystal layer is a nematic liquid crystal layer wherein the twist angle between the transparent substrates is about 90xc2x0 or 70-80xc2x0, the duty ratio for driving the nematic liquid crystal layer is 1/1-1/4.
In accordance with an aspect of the present invention, the liquid crystal layer is a nematic liquid crystal layer having a twist angle of 180-270xc2x0 between the transparent substrates; a retardation film is disposed between at least one of the polarizers and a transparent substrate opposing thereto, and said pair of polarizers are arranged so that light shielding and light transmitting are switched by applying a voltage.
In accordance with an aspect of the present invention, the liquid crystal layer is a ferrodielectric liquid crystal or an anti-ferrodielectric liquid crystal; said pair of polarizers are arranged so that their polarizing axes intersect at a substantially right angle, and light shielding and light transmitting are switched by applying a voltage.
Since the adhesive properties of the light shielding layer to the transparent substrate is excellent, it can be provided between the peripheral sealing member and the transparent substrate and also the outer edge of the shielding layer is inside of the sealing member.
As a characteristic feature of the present invention, when a TN (Twisted Nematic) system is used for displaying a negative display, a nematic liquid crystal added with a dichroic dye is used to improve the contrast ratio.
As the illumination means, it is preferred to use a white color light source such as a white LED (light emission diode) having the spectra of three colors of R, G and B or a white CCT although a tungsten lamp, a Xenon: lamp, EL or the like, which has conventionally been used, is usable.
Further, the resinous material having black pigment may be a resinous material obtained by polymerizing a resin composition including an acid adduct of alkali-soluble epoxy acrylate including insulating carbon. Further, the resinous material for the light transmitting layer may be a resinous material obtained by polymerizing a resin composition including an acid adduct of alkali-soluble epoxy acrylate. Further, the light shielding layer is provided below a lower portion of the periphery sealing member, and the edge portion of the light shielding layer is within the width of the periphery sealing member over the entire periphery.